Sedimentation in the Amazon Sedimentation in the Amazon River Delta and Nearby River Delta and Nearby

Continental ShelfContinental Shelf

Karen KoyKaren Koy

Geology 455Geology 455

IntroductionIntroduction

The Amazon River contributes 20% of the The Amazon River contributes 20% of the global runoff into oceans (Degens global runoff into oceans (Degens et alet al., ., 1991)1991)

Its annual discharge is 6,300 kmIts annual discharge is 6,300 km33/yr /yr (Milliman and Meade, 1983)(Milliman and Meade, 1983)

Makes an important contribution to the Makes an important contribution to the sedimentation and geomorphology of the sedimentation and geomorphology of the northeastern coast of South Americanortheastern coast of South America

Summary of the current literatureSummary of the current literature Make some predictions about the future of Make some predictions about the future of

this regionthis region

IntroductionIntroduction

From Nittrouer et al., 1995

The Amazon The Amazon River flows from River flows from the Andean the Andean mountain range mountain range east across east across South America South America as an as an anastomozing anastomozing river, with river, with numerous fluvial numerous fluvial islands and islands and channel bars channel bars finally ending at finally ending at the Atlantic the Atlantic Ocean (Ocean (Hoorn, Hoorn, 1994)1994)..

Amazon River MouthAmazon River Mouth

From Warne et al, 2002

The river mouth is The river mouth is fixed in place by a fixed in place by a graben (Dunne graben (Dunne et alet al., ., 1998)1998)

Characterized by Characterized by estuarine-like estuarine-like processes (Warne processes (Warne et et alal., 2002; Gibbs, 1969; ., 2002; Gibbs, 1969; Gibbs and Konwar, Gibbs and Konwar, 1986)1986)

Tidal influence can be Tidal influence can be seen ~100 km inland seen ~100 km inland (Hoorn, 1994)(Hoorn, 1994)

Amazon River MouthAmazon River Mouth The deepest portions reach >40m deep, yet The deepest portions reach >40m deep, yet

there is no saltwater intrusion (Gibbs, 1969; there is no saltwater intrusion (Gibbs, 1969; Gibbs and Konwar, 1986) Gibbs and Konwar, 1986)

The freshwater lens extends 120 km offshore The freshwater lens extends 120 km offshore (Gibbs, 1969)(Gibbs, 1969)

From Gibbs & Konwar, 1986

Sediment DischargeSediment Discharge

Sediment discharge is 1.2x10Sediment discharge is 1.2x1099 m m33 annually (Nittrouer annually (Nittrouer et alet al., 1995) ., 1995)

1-2% is bedload (Warne 1-2% is bedload (Warne et alet al., 2002)., 2002) 85-95% is suspended silt and clay 85-95% is suspended silt and clay

(Warne (Warne et alet al., 2002; Gibbs and ., 2002; Gibbs and Konwar, 1986) Konwar, 1986)

Mean grain size of 4μm (Gibbs and Mean grain size of 4μm (Gibbs and Konwar, 1986)Konwar, 1986)

Sediment DischargeSediment Discharge

Eighty-four percent of dissolved salts and Eighty-four percent of dissolved salts and solids sampled were nearly identical to the solids sampled were nearly identical to the Andean sediments (Gibbs, 1967)Andean sediments (Gibbs, 1967)

Suspended sediments come predominantly Suspended sediments come predominantly from the Andes Mountains, which cover only from the Andes Mountains, which cover only 12% of the total drainage basin area (McDaniel 12% of the total drainage basin area (McDaniel et alet al., 1996; Gibbs, 1967)., 1996; Gibbs, 1967)

The relief and physical weathering of the The relief and physical weathering of the Andes Mountains appears to be a major Andes Mountains appears to be a major control on the geochemistry of the Amazon control on the geochemistry of the Amazon River (Gibbs, 1967)River (Gibbs, 1967)

Sediment DischargeSediment Discharge Even though it covers a majority of the Even though it covers a majority of the

drainage basin area, the lower tropical drainage basin area, the lower tropical rainforest contributes very little to the rainforest contributes very little to the suspended load of the Amazon River suspended load of the Amazon River (McDaniel (McDaniel et alet al., 1996; Gibbs, 1967)., 1996; Gibbs, 1967)

It is likely the high level of vegetation in the It is likely the high level of vegetation in the rainforests retards surface erosionrainforests retards surface erosion

The Amazon River channels and tributaries The Amazon River channels and tributaries meander and migrate, producing elaborate meander and migrate, producing elaborate scrollwork geomorphology (Mertes scrollwork geomorphology (Mertes et alet al., ., 1996), reworking the fluviolacustrine 1996), reworking the fluviolacustrine sedimentssediments

The reworked sediments may comprise the The reworked sediments may comprise the majority of the bedloadmajority of the bedload

The composition of the Amazon River The composition of the Amazon River bedload has yet to be investigatedbedload has yet to be investigated

Amazon River DeltaAmazon River Delta A subaqueous deposit located seaward A subaqueous deposit located seaward

of the river mouth on the continental of the river mouth on the continental shelf (Nittrouer shelf (Nittrouer et alet al., 1995; Vital and ., 1995; Vital and Stattegger, 2000; McDaniel Stattegger, 2000; McDaniel et alet al., ., 1996; Gibbs and Konwar, 1986)1996; Gibbs and Konwar, 1986)

50 km wide at the proximal end, 300 50 km wide at the proximal end, 300 km wide distally, and 650 km long, km wide distally, and 650 km long, running northwest and parallel to the running northwest and parallel to the coast (de Alencar Costa and coast (de Alencar Costa and Figueiredo, 1998)Figueiredo, 1998)

The delta plain contains estuarine The delta plain contains estuarine islands (Warne islands (Warne et alet al., 2002)., 2002)

During a previous lowstand, sediments During a previous lowstand, sediments were deposited in the Amazon Fan were deposited in the Amazon Fan located in the deep ocean (McDaniel located in the deep ocean (McDaniel et et alal., 1996)., 1996)

From Warne et al, 2002

Amazon River DeltaAmazon River Delta

Underlying the delta Underlying the delta is a transgressive is a transgressive sand layer, which is sand layer, which is the top of a series the top of a series of mud and sand of mud and sand layers 25-30 m thick layers 25-30 m thick (Figueiredo and (Figueiredo and Nittrouer, 1995; de Nittrouer, 1995; de Alencar Costa and Alencar Costa and Figueiredo, 1998)Figueiredo, 1998)

From Figueireda and Nittrouer, 1995

Amazon River DeltaAmazon River Delta

The topmost sandy layer increases in The topmost sandy layer increases in thickness towards the river mouth thickness towards the river mouth (Alexander (Alexander et alet al., 1986)., 1986)

The mud layers are interpreted as The mud layers are interpreted as regressive, prodelta sedimentsregressive, prodelta sediments

The sand layers are interpreted as The sand layers are interpreted as transgressive strata from delta transgressive strata from delta progradation (Figueiredo and Nittrouer, progradation (Figueiredo and Nittrouer, 1995)1995)

Amazon River DeltaAmazon River Delta

Modern sediment accumulation occurs in an Modern sediment accumulation occurs in an offshore depocenter >30 m deep, which includes offshore depocenter >30 m deep, which includes the outer topset and foreset strata (Allison the outer topset and foreset strata (Allison et alet al., ., 2000; Nittrouer 2000; Nittrouer et alet al., 1995; Alexander ., 1995; Alexander et alet al., 1986)., 1986)

The sediment accumulation rate increases seaward The sediment accumulation rate increases seaward (Kuehl (Kuehl et alet al., 1986)., 1986)

Sediment accumulation rates in the foreset region: Sediment accumulation rates in the foreset region: 5.0-7.3 cm/yr (Dukat and Kuehl, 1995) 5.0-7.3 cm/yr (Dukat and Kuehl, 1995) 8-10 cm/yr (Alexander 8-10 cm/yr (Alexander et alet al., 1986) ., 1986) 10.3 cm/yr (Kuehl 10.3 cm/yr (Kuehl et alet al., 1986)., 1986)

Amazon River DeltaAmazon River Delta

Suspended Suspended sediment from the sediment from the river plume river plume flocculates and flocculates and settles out along settles out along the delta and the delta and northwest along the northwest along the continental shelf continental shelf with decreasing with decreasing grain size distally grain size distally (Gibbs and Konwar, (Gibbs and Konwar, 1986 ; de Alencar 1986 ; de Alencar Costa and Costa and Figueiredo, 1998).Figueiredo, 1998).

From Gibbs & Konwar, 1986

The Continental ShelfThe Continental Shelf

Northwest of the Amazon River is the longest Northwest of the Amazon River is the longest continuous accretionary mud coastline in the continuous accretionary mud coastline in the world (Allison world (Allison et alet al., 2000; Wells and ., 2000; Wells and Coleman, 1981; Rine and Ginsburg, 1985)Coleman, 1981; Rine and Ginsburg, 1985)

The Continental ShelfThe Continental Shelf

The Amazon sediment plume travels northwest The Amazon sediment plume travels northwest along the coast in a 20-40 km wide zone along the coast in a 20-40 km wide zone (Froidefond (Froidefond et alet al., 1988)., 1988)

Travel at a rate of 40-80 cm/s (Nittrouer Travel at a rate of 40-80 cm/s (Nittrouer et alet al., ., 1995)1995)

These sediments can still be found up to 1600 These sediments can still be found up to 1600 km updrift (Froidefond km updrift (Froidefond et alet al., 1988; Gibbs, ., 1988; Gibbs, 1977)1977)

The sediments from the Amazon River travel in The sediments from the Amazon River travel in two basic forms:two basic forms: as suspended sedimentas suspended sediment as mudbanksas mudbanks

The Continental ShelfThe Continental Shelf

~100 million m~100 million m33/yr suspended sediments travel /yr suspended sediments travel up the coast (Froidefond up the coast (Froidefond et alet al., 1988; Wells and ., 1988; Wells and Coleman, 1981Coleman, 1981

The North Brazilian Current is the primary force The North Brazilian Current is the primary force moving the sediment plume (Nittrouer moving the sediment plume (Nittrouer et alet al., ., 1995; Wells and Coleman, 1981; Gibbs, 1969)1995; Wells and Coleman, 1981; Gibbs, 1969)

Easterly trade winds combine with the North Easterly trade winds combine with the North Brazilian Current to reach a maximum migration Brazilian Current to reach a maximum migration rate during the months of August and September rate during the months of August and September (Nittrouer (Nittrouer et alet al., 1995; Wells and Coleman, 1981; ., 1995; Wells and Coleman, 1981; Rine and Ginsburg, 1985; Gibbs, 1969)Rine and Ginsburg, 1985; Gibbs, 1969)

The Continental ShelfThe Continental Shelf >150 million m>150 million m33/yr sediment travel alongshore in the /yr sediment travel alongshore in the

form of mudbanks (Froidefond form of mudbanks (Froidefond et alet al., 1988; Allison ., 1988; Allison et et alal., 2000; Wells and Coleman, 1981). ., 2000; Wells and Coleman, 1981).

Mudbanks are volumetrically stable, can travel over 900 Mudbanks are volumetrically stable, can travel over 900 m/yr (Froidefond m/yr (Froidefond et alet al., 1988; Wells and Coleman, 1981)., 1988; Wells and Coleman, 1981)

Remain subaqueous (Vital and Stattegger, 2000)Remain subaqueous (Vital and Stattegger, 2000) Linear, shore-attached migrating shoals are similar to Linear, shore-attached migrating shoals are similar to

features found on sandy coastlinesfeatures found on sandy coastlines Distributed every 30-60 km northwest along the coast, Distributed every 30-60 km northwest along the coast,

have <5m relief, and thin towards the shore and have <5m relief, and thin towards the shore and seaward (Wells and Coleman, 1981)seaward (Wells and Coleman, 1981)

They act as barriers to wave energy at low tide, They act as barriers to wave energy at low tide, trapping and storing finer-grained sediments (Wells and trapping and storing finer-grained sediments (Wells and Coleman, 1981)Coleman, 1981)

The Continental ShelfThe Continental Shelf

The coastline adjacent to the The coastline adjacent to the Amazon River mouth is eroding and Amazon River mouth is eroding and contributing sediment to the updrift contributing sediment to the updrift mudbanks (Allison mudbanks (Allison et alet al., 2000)., 2000)

A shoreline retreat rate of 0.5-1.0 A shoreline retreat rate of 0.5-1.0 m/yr, due to tidal and wave induced m/yr, due to tidal and wave induced currents (Allison currents (Allison et alet al., 1995)., 1995)

The Continental ShelfThe Continental Shelf

The shelf sedimentation rate reaches its The shelf sedimentation rate reaches its maximum in the upper tidal zone, and maximum in the upper tidal zone, and increases seaward across the shelf (Kuehl increases seaward across the shelf (Kuehl et et alal., 1986; Allison ., 1986; Allison et alet al., 1995)., 1995)

There is an overall upward/seaward There is an overall upward/seaward progradation of the coastal plain (Nittrouer progradation of the coastal plain (Nittrouer et alet al., 1995)., 1995)

Contrary to previous belief, the fine-grained Contrary to previous belief, the fine-grained sediments along this muddy coastline are sediments along this muddy coastline are deposited by high-energy waves and deposited by high-energy waves and currents (Rine and Ginsburg, 1985).currents (Rine and Ginsburg, 1985).

ConclusionsConclusions The majority of sediment carried by the The majority of sediment carried by the

Amazon River is suspended sediment derived Amazon River is suspended sediment derived from the Andes Mountains to the westfrom the Andes Mountains to the west

It is deposited in a subaqueous delta, and It is deposited in a subaqueous delta, and northwest along the continental shelf, fining northwest along the continental shelf, fining with increasing distance from the river mouthwith increasing distance from the river mouth

The topset and foreset regions of the delta The topset and foreset regions of the delta have the highest sedimentation rates, causing have the highest sedimentation rates, causing the delta to prograde outward and upwardthe delta to prograde outward and upward

The coastline is being eroded near the river The coastline is being eroded near the river mouth, and muddy sediments are deposited mouth, and muddy sediments are deposited updrift along the continental shelf as updrift along the continental shelf as migrating mud shoalsmigrating mud shoals

Conclusions: Further Conclusions: Further InvestigationInvestigation

The composition and origins of the The composition and origins of the bedloadbedload

Why so little sediment is derived Why so little sediment is derived from the non-Andean source areasfrom the non-Andean source areas

The mechanism that would cause or The mechanism that would cause or maintain estuarine conditions in the maintain estuarine conditions in the Amazon River mouthAmazon River mouth

Conclusions: PredictionsConclusions: Predictions

If the current data are accurate, and the If the current data are accurate, and the sedimentation and erosion patterns along sedimentation and erosion patterns along the coastal shelf do not change:the coastal shelf do not change:

The sediments from the Amazon River, and The sediments from the Amazon River, and those being eroded from the nearby shelf, those being eroded from the nearby shelf, will continue moving northwest along the will continue moving northwest along the coastcoast

The muddy coastline will continue to The muddy coastline will continue to migrate even further north, possibly migrate even further north, possibly extending past the Orinoco River mouthextending past the Orinoco River mouth

The Amazon River delta will most likely The Amazon River delta will most likely remain subaqueousremain subaqueous

ConclusionsConclusions

If sea-level were to fall:If sea-level were to fall: The delta and mudbanks would The delta and mudbanks would

become subaerialbecome subaerial

If sea-level achieves a lowstand:If sea-level achieves a lowstand: Sediments from the Amazon River Sediments from the Amazon River

may once again be deposited in the may once again be deposited in the deep marine Amazon Fan deep marine Amazon Fan